The warning is in effect from 3 a.m. through 10 a.m. Friday. It covers Harford, northern Baltimore, Carroll and Howard counties and points west.

Forecasters cautioned that unprotected plants could be killed or damaged. For those who don't have gardens or crops to worry about, the warning is a sign that it might soon or already be time to turn on the heat.

Downtown, temperatures had reached 55 degrees by 2 p.m. The urban heat island effect kept Baltimore City and suburbs around it in southern Baltimore County out of the freeze warning zone, with overnight lows forecast in the upper 30s.

Partly cloudy skies and some sunshine are again expected on Friday, and another dip toward freezing temperatures is expected overnight into Saturday morning.

For the weekend, highs are forecast in the mid- to upper-60s with lows in the upper 30s and lower 40s and mostly clear skies Saturday but increasing clouds Sunday.

Historic flooding and the coldest temperatures in decades made weather headlines in Baltimore in 2014. Rainfall records were set in April and August, and after a frigid winter, the summer was comfortable and fall cool. The year is expected to be the coolest and wettest here in years.